📝 Posted:

2023-05-01 23:56 UTC

🚚 Summary of:

P0238, P0239

⌨ Commits:

(Website) 4698397...edf2926, c5e51e6...P0239

💰 Funded by:

Ember2528

🏷 Tags:

website+ th01+ meta+ anniversary-edition- debloating+ master.lib+ cutscene+ animation+ debug+

Stripe is now properly integrated into this website as an alternative to PayPal! Now, you can also financially support the project if PayPal doesn't work for you, or if you prefer using a provider out of Stripe's greater variety. It's unfortunate that I had to ship this integration while the store is still sold out, but the Shuusou Gyoku OpenGL backend has turned out way too complicated to be finished next to these two pushes within a month. It will take quite a while until the store reopens and you all can start using Stripe, so I'll just link back to this blog post when it happens.

Integrating Stripe wasn't the simplest task in the world either. At first, the Checkout API seems pretty friendly to developers: The entire payment flow is handled on the backend, in the server language of your choice, and requires no frontend JavaScript except for the UI feedback code you choose to write. Your backend API endpoint initiates the Stripe Checkout session, answers with a redirect to Stripe, and Stripe then sends a redirect back to your server if the customer completed the payment. Superficially, this server-based approach seems much more GDPR-friendly than PayPal, because there are no remote scripts to obtain consent for. In reality though, Stripe shares much more potential personal data about your credit card or bank account with a merchant, compared to PayPal's almost bare minimum of necessary data.
It's also rather annoying how the backend has to persist the order form information throughout the entire Checkout session, because it would otherwise be lost if the server restarts while a customer is still busy entering data into Stripe's Checkout form. Compare that to the PayPal JavaScript SDK, which only POSTs back to your server after the customer completed a payment. In Stripe's case, more JavaScript actually only makes the integration harder: If you trigger the initial payment HTTP request from JavaScript, you will have to improvise a bit to avoid the CORS error when redirecting away to a different domain.

But sure, it's all not too bad… for regular orders at least. With subscriptions, however, things get much worse. Unlike PayPal, Stripe kind of wants to stay out of the way of the payment process as much as possible, and just be a wrapper around its supported payment methods. So if customers aren't really meant to register with Stripe, how would they cancel their subscriptions?
Answer: Through the… merchant? Which I quite dislike in principle, because why should you have to trust me to actually cancel your subscription after you requested it? It also means that I probably should add some sort of UI for self-canceling a Stripe subscription, ideally without adding full-blown user accounts. Not that this solves the underlying trust issue, but it's more convenient than contacting me via email or, worse, going through your bank somehow. Here is how my solution works:

• When setting up a Stripe subscription, the server will generate a random ID for authentication. This ID is then used as a salt for a hash of the Stripe subscription ID, linking the two without storing the latter on my server.
• The thank you page, which is parameterized with the Stripe Checkout session ID, will use that ID to retrieve the subscription ID via an API call to Stripe, and display it together with the above salt. This works indefinitely – contrary to what the expiry field in the Checkout session object suggests, Stripe sessions are indeed stored forever. After all, Stripe also displays this session information in a merchant's transaction log with an excessive amount of detail. It might have been better to add my own expiration system to these pages, but this had been taking long enough already. For now, be aware that sharing the link to a Stripe thank you page is equivalent to sharing your subscription cancellation password.
• The salt is then used as the key for a subscription management page. To cancel, you visit this page and enter the Stripe subscription ID to confirm. The server then checks whether the salt and subscription ID pair belong to each other, and sends the actual cancellation request back to Stripe if they do.

I might have gone a bit overboard with the crypto there, but I liked the idea of not storing any of the Stripe session IDs in the server database. It's not like that makes the system more complex anyway, and it's nice to have a separate confirmation step before canceling a subscription.

But even that wasn't everything I had to keep in mind here. Once you switch from test to production mode for the final tests, you'll notice that certain SEPA-based payment providers take their sweet time to process and activate new subscriptions. The Checkout session object even informs you about that, by including a payment status field. Which initially seems just like another field that could indicate hacking attempts, but treating it as such and rejecting any unpaid session can also reject perfectly valid subscriptions. I don't want all this control… 🥲
Instead, all I can do in this case is to tell you about it. In my test, the Stripe dashboard said that it might take days or even weeks for the initial subscription transaction to be confirmed. In such a case, the respective fraction of the cap will unfortunately need to remain red for that entire time.

And that was 1½ pushes just to replicate the basic functionality of a simple PayPal integration with the simplest type of Stripe integration. On the architectural site, all the necessary refactoring work made me finally upgrade my frontend code to TypeScript at least, using the amazing esbuild to handle transpilation inside the server binary. Let's see how long it will now take for me to upgrade to SCSS…

With the new payment options, it makes sense to go for another slight price increase, from up to per push. The amount of taxes I have to pay on this income is slowly becoming significant, and the store has been selling out almost immediately for the last few months anyway. If demand remains at the current level or even increases, I plan to gradually go up to by the end of the year.
📝 As 📝 usual, I'm going to deliver existing orders in the backlog at the value they were originally purchased at. Due to the way the cap has to be calculated, these contributions now appear to have increased in value by a rather awkward 13.33%.

This left ½ of a push for some more work on the TH01 Anniversary Edition. Unfortunately, this was too little time for the grand issue of removing byte-aligned rendering of bigger sprites, which will need some additional blitting performance research. Instead, I went for a bunch of smaller bugfixes:

• ANNIV.EXE now launches ZUNSOFT.COM if MDRV98 wasn't resident before. In hindsight, it's completely obvious why this is the right thing to do: Either you start ANNIV.EXE directly, in which case there's no resident MDRV98 and you haven't seen the ZUN Soft logo, or you have made a single-line edit to GAME.BAT and replaced op with anniv, in which case MDRV98 is resident and you have seen the logo. These are the two reasonable cases to support out of the box. If you are doing anything else, it shouldn't be that hard to adjust though?

  You might be wondering why I didn't just include all code of ZUNSOFT.COM inside ANNIV.EXE together with the rest of the game. The reason: ZUNSOFT.COM has almost nothing in common with regular TH01 code. While the rest of TH01 uses the custom image formats and bad rendering code I documented again and again during its RE process, ZUNSOFT.COM fully relies on master.lib for everything about the bouncing-ball logo animation. Its code is much closer to TH02 in that respect, which suggests that ZUN did in fact write this animation for TH02, and just included the binary in TH01 for consistency when he first sold both games together at Comiket 52. Unlike the 📝 various bad reasons for splitting the PC-98 Touhou games into three main executables, it's still a good idea to split off animations that use a completely different set of rendering and file format functions. Combined with all the BFNT and shape rendering code, ZUNSOFT.COM actually contains even more unique code than OP.EXE, and only slightly less than FUUIN.EXE.

• The optional AUTOEXEC.BAT is now correctly encoded in Shift-JIS instead of accidentally being UTF-8, fixing the previous mojibake in its final ECHO line.

• The command-line option that just adds a stage selection without other debug features (anniv s) now works reliably.

  This one's quite interesting because it only ever worked because of a ZUN bug. From a superficial look at the code, it shouldn't: While the presence of an 's' branch proves that ZUN had such a mode during development, he nevertheless forgot to initialize the debug flag inside the resident structure within this branch. This mode only ever worked because master.lib's resdata_create() function doesn't clear the resident structure after allocation. If anything on the system previously happened to write something other than 0x00, 0x01, or 0x03 to the specific byte that then gets repurposed as the debug mode flag, this lack of initialization does in fact result in a distinct non-test and non-debug stage selection mode.
  This is what happens on a certain widely circulated .HDI copy of TH01 that boots MS-DOS 3.30C. On this system, the memory that master.lib will allocate to the TH01 resident structure was previously used by DOS as stack for its kernel, which left the future resident debug flag byte at address 9FF6:0012 at a value of 0x12. This might be the entire reason why game s is even widely documented to trigger a stage selection to begin with – on the widely circulated TH04 .HDI that boots MS-DOS 6.20, or on DOSBox-X, the s parameter doesn't work because both DOS systems leave the resident debug flag byte at 0x00. And since ANNIV.EXE pushes MDRV98 into that area of conventional DOS RAM, anniv s previously didn't work even on MS-DOS 3.30C.

• Both bugs in the 📝 1×1 particle system during the Mima fight have been fixed. These include the off-by-one error that killed off the very first particle on the 80^th frame and left it in VRAM, and, just like every other entity type, a replacement of ZUN's EGC unblitter with the new pixel-perfect and fast one. Until I've rearchitected unblitting as a whole, the particles will now merely rip barely visible 1×1 holes into the sprites they overlap.

• The 📝 score popups for flipped cards are now displayed without the two frames of flicker.

• The bomb value shown in the lowest line of the in-game debug mode output is now right-aligned together with the rest of the values. This ensures that the game always writes a consistent number of characters to TRAM, regardless of the magnitude of the bomb value, preventing the seemingly wrong timer values that appeared in the original game whenever the value of the bomb variable changed to a lower number of digits:

• Finally, I've streamlined VRAM page access changes, which allowed me to consistently replace ZUN's expensive function call with the optimal two inlined x86 instructions. Interestingly, this change alone removed 2 KiB from the binary size, which is almost all of the difference between 📝 the P0234-1 release and this one. Let's see how much longer we can make each new release of ANNIV.EXE smaller than the previous one.

The final point, however, raised the question of what we're now going to do about 📝 a certain issue in the 地獄/Jigoku Bad Ending. ZUN's original expensive way of switching the accessed VRAM page was the main reason behind the lag frames on slower PC-98 systems, and search-replacing the respective function calls would immediately get us to the optimized version shown in that blog post. But is this something we actually want? If we wanted to retain the lag, we could surely preserve that function just for this one instance…
The discovery of this issue predates the clear distinction between bloat, quirks, and bugs, so it makes sense to first classify what this issue even is. The distinction comes all down to observability, which I defined as changes to rendered frames between explicitly defined frame boundaries. That alone would be enough to categorize any cause behind lag frames as bloat, but it can't hurt to be more explicit here.

Therefore, I now officially judge observability in terms of an infinitely fast PC-98 that can instantly render everything between two explicitly defined frames, and will never add additional lag frames. If we plan to port the games to faster architectures that aren't bottlenecked by disappointing blitter chips, this is the only reasonable assumption to make, in my opinion: The minimum system requirements in the games' README files are minimums, after all, not recommendations. Chasing the exact frame drop behavior that ZUN must have experienced during the time he developed these games can only be a guessing game at best, because how can we know which PC-98 model ZUN actually developed the games on? There might even be more than one model, especially when it comes to TH01 which had been in development for at least two years before ZUN first sold it. It's also not like any current PC-98 emulator even claims to emulate the specific timing of any existing model, and I sure hope that nobody expects me to import a bunch of bulky obsolete hardware just to count dropped frames.

That leaves the tearing, where it's much more obvious how it's a bug. On an infinitely fast PC-98, the ドカーン frame would never be visible, and thus falls into the same category as the 📝 two unused animations in the Sariel fight. With only a single unconditional 2-frame delay inside the animation loop, it becomes clear that ZUN intended both frames of the animation to be displayed for 2 frames each:

TH01 Anniversary Edition, version P0239 2023-05-01-th01-anniv.zip

Next up: Taking the oldest still undelivered push and working towards TH04 position independence in preparation for multilingual translations. The Shuusou Gyoku OpenGL backend shouldn't take that much longer either, so I should have lots of stuff coming up in May afterward.

📝 Posted:

2023-03-14

🏷 Tags:

rec98+ th01+ anniversary-edition- gameplay+ bullet+ boss+ sariel+ glitch+

Turns out I was not quite done with the TH01 Anniversary Edition yet. You might have noticed some white streaks at the beginning of Sariel's second form, which are in fact a bug that I accidentally added to the initial release.
These can be traced back to a quirk I wasn't aware of, and hadn't documented so far. When defeating Sariel's first form during a pattern that spawns pellets, it's likely for the second form to start with additional pellets that resemble the previous pattern, but come out of seemingly nowhere. This shouldn't really happen if you look at the code: Nothing outside the typical pattern code spawns new pellets, and all existing ones are reset before the form transition…

Except if they're currently showing the 10-frame delay cloud animation , activated for all pellets during the symmetrical radial 2-ring pattern in Phase 2 and left activated for the rest of the fight. These pellets will continue their animation after the transition to the second form, and turn into regular pellets you have to dodge once their animation completed.

By itself, this is just one more quirk to keep in mind during refactoring. It only turned into a bug in the Anniversary Edition because the game tracks the number of living pellets in a separate counter variable. After resetting all pellets, this counter is simply set to 0, regardless of any delay cloud pellets that may still be alive, and it's merely incremented or decremented when pellets are spawned or leave the playfield.
In the original game, this counter is only used as an optimization to skip spawning new pellets once the cap is reached. But with batched EGC-accelerated unblitting, it also makes sense to skip the rather costly setup and shutdown of the EGC if no pellets are active anyway. Except if the counter you use to check for that case can be 0 even if there are pellets alive, which consequently don't get unblitted…
There is an optimal fix though: Instead of unconditionally resetting the living pellet counter to 0, we decrement it for every pellet that does get reset. This preserves the quirk and gives us a consistently correct counter, allowing us to still skip every unnecessary loop over the pellet array.

Ultimately, this was a harmless bug that didn't affect gameplay, but it's still something that players would have probably reported a few more times. So here's a free bugfix:

TH01 Anniversary Edition, version P0234-1 2023-03-14-th01-anniv.zip

Thanks to mu021 for reporting this issue and providing helpful videos to identify the cause!

📝 Posted:

2023-03-05 12:56 UTC

🚚 Summary of:

P0229, P0230, P0231, P0232, P0233, P0234

⌨ Commits:

6370f96...d535d87, d535d87...ca523b4, ca523b4...05a49b9, f7ef7f8...abeaf85, abeaf85...dbc5b51, dd2265c...12f29c6

💰 Funded by:

Ember2528, [Anonymous]

🏷 Tags:

rec98+ th01+ pc98+ tcc+ debloating+ file-format+ anniversary-edition- master.lib+ bullet+

128 commits! Who would have thought that the ideal first release of the TH01 Anniversary Edition would involve so much maintenance, and raise so many research questions? It's almost as if the real work only starts after the 100% finalization mark… Once again, I had to steal some funding from the reserved JIS trail word pushes to cover everything I liked to research, which means that the next towards the anything goal will repay this debt. Luckily, this doesn't affect any immediate plans, as I'll be spending March with tasks that are already fully funded.

So, how did this end up so massive? The list of things I originally set out to do was pretty short:

1. Build entire game into single executable
2. Fix rendering issues in the one or two most important parts of the game for a good initial impression

But even the first point already started with tons of little cleanup commits. A part of them can definitely be blamed on the rush to hit the 100% decompilation mark before the 25^th anniversary last August. However, all the structural changes that I can't commit to master reveal how much of a mess the TH01 codebase actually is.
Merging the executables is mainly difficult because of all the inconsistencies between REIIDEN.EXE and FUUIN.EXE. The worst parts can be found in the REYHI*.DAT format code and the High Score menu, but the little things are just as annoying, like how the current score is an unsigned variable in REIIDEN.EXE, but a signed one in FUUIN.EXE. If it takes me this long and this many commits just to sort out all of these issues, it's no wonder that the only thing I've seen being done with this codebase since TH01's 100% decompilation was a single porting attempt that ended in a rather quick ragequit.
So why are we merging the executables in preparation for the Anniversary Edition, and not waiting with it until we start doing ports?

• Distributing and updating one executable is cleaner than doing the same with three, especially as long as installation will still involve manually dropping the new binary into the game directory.
• The Anniversary Edition won't be the only fork binary. We are already going to start out with a separate DEBLOAT.EXE that contains only the bloat removal changes without any bug fixes, and spaztron64 will probably redo his seizure-less edition. We don't want to clutter the game directory with three binaries for each of these fork builds, and we especially don't want to remember things like oh, but this fork only modifies REIIDEN.EXE…
• All forks should run side-by-side with the original game. During the time I was maintaining thcrap, I've had countless bug reports of people assuming that thcrap was responsible for bugs that were present in the original game, and the same is certain to happen with the Anniversary Edition. Separate binaries will make it easier for everyone to check where these bugs came from.
• Also, I'd like to make a point about how bloated the original three-executable structure really is, since I've heard people defending it as neat software architecture. Really, even in Real Mode where you typically want to use as little of the 640 KiB of conventional memory as possible, you don't want to split your game up like this.

The game actually is so bloated that the combined binary ended up smaller than the original REIIDEN.EXE. If all you see are the file sizes of the original three executables, this might look like a pretty impressive feat. Like, how can we possibly get 407,812 bytes into less than 238,612 bytes, without using compression?
If you've ever looked at the linker map though, it's not at all surprising. Excluding the aforementioned inconsistencies that are hard to quantify, OP.EXE and FUUIN.EXE only feature 5,767 and 6,475 bytes of unique code and data, respectively. All other code in these binaries is already part of REIIDEN.EXE, with more than half of the size coming from the Borland C++ runtime. The single worst offender here is the C++ exception handler that Borland forces onto every non-.COM binary by default, which alone adds 20,512 bytes even if your binary doesn't use C++ exceptions.
On a more hilarious note, this single line is responsible for pulling another unnecessary 14,242 bytes into OP.EXE and FUUIN.EXE. This floating-point multiplication is completely unnecessary in this context because all possible parameters are integers, but it's enough for Turbo C++ and TLINK to pull in the entire x87 FPU emulation machinery. These two binaries don't even draw lines, but since this function is part of the general graphics code translation unit and contains other functions that these binaries do need, TLINK links in the entire thing. Maybe, multiple executables aren't the best choice either if you use a linker that can't do dead code elimination…

Since the 📝 Orb's physics do turn the entire precision of a double variable into gameplay effects, it's not feasible to ever get rid of all FPU code in TH01. The exception handler, however, can be removed, which easily brings the combined binary below the size of the original REIIDEN.EXE. Compiling all code with a single set of compiler optimization flags, including the more x86-friendly pascal calling convention, then gets us a few more KB on top. As does, of course, removing unused code: The only remaining purpose of features such as 📝 resident palettes is to potentially make porting more difficult for anyone who doesn't immediately realize that nothing in the game uses these functions.
Technically, all unused code would be bloat, but for now, I'm keeping the parts that may tell stories about the game's development history (such as unused effects or the 📝 mouse cursor), or that might help with debugging. Even with that in mind, I've only scratched the surface when it comes to bloat removal, and the binary is only going to get smaller from here. A lot smaller.

If only we now could start MDRV98 from this new combined binary, we wouldn't need a second batch file either…

Which brings us to the first big research question of this delivery. Using the C spawn() function works fine on this compiler, so spawn("MDRV98.COM") would be all we need to do, right? Except that the game crashes very soon after that subprocess returned.
So it's not going to be that easy if the spawned process is a TSR. But why should this be a problem? Let's take a look at the DOS heap, and how DOS lays out processes in conventional memory if we launch the game regularly through GAME.BAT:

The batch file starts MDRV98 first, which will therefore end up below the game in conventional memory. This is perfect for a TSR: The program can resize itself arbitrarily before returning to DOS, and the rest of memory will be left over for the game. If we assume such a layout, a DOS program can implement a custom memory allocator in a very simple way, as it only has to search for free memory in one direction – and this is exactly how Borland implemented the C heap for functions like malloc() and free(), and the C++ new and delete operators.
But if we spawn MDRV98 after starting TH01, well…

MDRV98 will spawn in the next free memory location, allocate itself, return to TH01… which suddenly finds its C heap blocked from growing. As a result, the next big allocation will immediately fail with a rather misleading "out of memory" error.

So, what can we do about this? Still in a bloat removal mindset, my gut reaction was to just throw out Borland's C heap implementation, and replace it with a very thin wrapper around the DOS heap as managed by INT 21h, AH=48h/49h/4Ah. Like, why did these DOS compilers even bother with a custom allocator in the first place if DOS already comes with a perfectly fine native one? Using the native allocator would completely erase the distinction between TSR memory and game memory, and inherently allow the game to allocate beyond MDRV98.
I did in fact implement this, and noticed even more benefits:

• While DOS uses 16 bytes rather than Borland's 4 bytes for the control structure of each memory block, this larger size automatically aligns all allocations to 16-byte boundaries. Therefore, all allocation addresses would fit into 16-bit segment-only pointers rather than needing 32-bit far ones. On the Borland heap, the 4-byte header further limits regular far pointers to 65,532 bytes, forcing you into expensive huge pointers for bigger allocations.
• Debuggers in DOS emulators typically have features to show and manage the DOS heap. No need for custom debugging code.
• You can change the memory placement strategy to allocate from the top of conventional memory down to the bottom. This is how the games allocate their resident structures.

Ultimately though, the drawbacks became too significant. Most of them are related to the PC-98 Touhou games only ever creating a single DOS process, even though they contain multiple executables. Switching executables is done via exec(), which resizes a program's main allocation to match the new binary and then overwrites the old program image with the new one. If you've ever wondered why DOSBox-X only ever shows OP as the active process name in the title bar, you now know why. As far as DOS is concerned, it's still the same OP.EXE process rooted at the same segment, and exec() doesn't bother rewriting the name either. Most importantly though, this is how REIIDEN.EXE can launch into another REIIDEN.EXE process even if there are less than 238,612 bytes free when exec() is called, and without consuming more memory for every successive binary.
For now, ANNIV.EXE still re-exec()s itself at every point where the original game did, as ZUN's original code really depends on being reinitialized at boss and scene boundaries. The resulting accidental semi-hot reloading is also a useful property to retain during development.
So why is the DOS heap a bad idea for regular game allocation after all?

• Even DOS automatically releases all memory associated with a process during its termination. But since we keep running the same process until the player quits out of the main menu, we lose the C heap's implicit cleanup on exec(), and have to manually free all memory ourselves.
• Since the binary can be larger after hot reloading, we in fact have to allocate all regular memory using the last fit strategy. Otherwise, exec() fails to resize the program's main block for the same reason that crashed the game on our initial attempt to spawn("MDRV98.COM").
• Just like Borland's heap implementation, the DOS heap stores its control structures immediately before each allocation, forming a singly linked list. But since the entire OS shares this single list, corruptions from heap overflows also affect the whole system, and become much more disastrous. Theoretically, it might be possible to recover from them by forcibly releasing all blocks after the last correct one, or even by doing a brute-force search for valid memory control blocks, but in reality, DOS will likely just throw error code #7 (ERROR_ARENA_TRASHED) on the next memory management syscall, forcing a reboot.
  With a custom allocator, small corruptions remain isolated to the process. They can be even further limited if the process adds some padding between its last internal allocation and the end of the allocated DOS memory block; Borland's heap sort of does this as well by always rounding up the DOS block to a full KiB. All this might not make a difference in today's emulated and single-tasked usage, but would have back then when software was still developed inside IDEs running on the same system.
• TH01's debug mode uses heapcheck() and heapchecknode(), and reimplementing these on top of the DOS heap is not trivial. On the contrary, it would be the most complicated part of such a wrapper, by far.
• Finally, and most importantly for TH01 in particular: The observable effects of the 📝 test/debug mode HP bar heap corruption glitches are a direct result of Borland's C heap implementation.

I could release this DOS heap wrapper in unused form for another push if anyone's interested, but for now, I'm pretty happy with not actually using it in the games. Instead, let's stay with the Borland C heap, and find a way to push MDRV98 to the very top of conventional RAM. Like this:

Which is much easier said than done. It would be nice if we could just use the last fit allocation strategy here, but .COM executables always receive all free memory by default anyway, which eliminates any difference between the strategies.
But we can still change memory itself. So let's temporarily claim all remaining free memory, minus the exact amount we need for MDRV98, for our process. Then, the only remaining free space to spawn MDRV98 is at the exact place where we want it to be:

Now we only need to know how much memory to not temporarily allocate. First, we need to replicate the assumption that MDRV98's -M7 command-line parameter corresponds to a resident size of 23,552 bytes. This is not as bad as it seems, because the -M parameter explicitly has a KiB unit, and we can nicely abstract it away for the API.
The (env.) block though? Its minimum size equals the combined length of all environment variables passed to the process, but its maximum size is… not limited at all?! As in, DOS implementations can add and have historically added more free space because some programs insisted on storing their own new environment variables in this exact segment. DOSBox and DOSBox-X follow this tradition by providing a configuration option for the additional amount of environment space, with the latter adding 1024 additional bytes by default, y'know, just in case someone wants to compile FreeDOS on a slow emulator. It's not even worth sending a bug report for this specific case, because it's only a symptom of the fact that unexpectedly large program environment blocks can and will happen, and are to be expected in DOS land.
So thanks to this cruel joke, it's technically impossible to achieve what we want to do there. Hooray! The only thing we can kind of do here is an educated guess: Sum up the length of all environment variables in our environment block, compare that length against the allocated size of the block, and assume that the MDRV98 process will get as much additional memory as our process got. 🤷

The remaining hurdles came courtesy of some Borland C runtime implementation details. You would think that the temporary reallocation could even be done in pure C using the sbrk(), coreleft(), and brk() functions, but all values passed to or returned from these functions are inaccurate because they don't factor in the aforementioned KiB padding to the underlying DOS memory block. So we have to directly use the DOS syscalls after all. Which at least means that learning about them wasn't completely useless…
The final issue is caused inside Borland's spawn() implementation. The environment block for the child process is built out of all the strings reachable from C's environ pointer, which is what that FreeDOS build process should have used. Coalescing them into a single buffer involves yet another C heap allocation… and since we didn't report our DOS memory block manipulation back to the C heap, the malloc() call might think it needs to request more memory from DOS. This resets the DOS memory block back to its intended level, undoing our manipulation right before the actual INT 21h, AH=4Bh EXEC syscall. Or in short:

Manipulate DOS heap ➜ spawn() call ➜ _LoadProg() ➜ allocate and prepare environment block ➜ _spawn() ➜ DOS EXEC syscall

The obvious solution: Replace _LoadProg(), implement the coalescing ourselves, and do it before the heap manipulation. Fortunately, Borland's internal low-level _spawn() function is not static, so we can call it ourselves whenever we want to:

Allocate and prepare environment block ➜ manipulate DOS heap ➜ _spawn() call ➜ EXEC syscall

So yes, launching MDRV98 from C can be done, but it involves advanced witchcraft and is completely ridiculous. Launching external sound drivers from a batch file is the right way of doing things.
Fortunately, you don't have to rely on this auto-launching feature. You can still launch DEBLOAT.EXE or ANNIV.EXE from a batch file that launched MDRV98.COM before, and the binaries will detect this case and skip the attempt of launching MDRV98 from C. It's unlikely that my heuristic will ever break, but I definitely recommend replicating GAME.BAT just to be completely sure – especially for user-friendly repacks that don't want to include the original game anyway.
This is also why ANNIV.EXE doesn't launch ZUNSOFT.COM: The "correct" and stable way to launch ANNIV.EXE still involves a batch file, and I would say that expecting people to remove ZUNSOFT.COM from that file is worse than not playing the animation. It's certainly a debate we can have, though.

This deep dive into memory allocation revealed another previously undocumented bug in the original game. The RLE decompression code for the 東方靈異.伝 packfile contains two heap overflows, which are actually triggered by SinGyoku's BOSS1_3.BOS and Konngara's BOSS8_1.BOS. They only do not immediately crash the game when loading these bosses thanks to two implementation details of Borland's C heap.
Obviously, this is a bug we should fix, but according to the definition of bugs, that fix would be exclusive to the anniversary branch. Isn't that too restrictive for something this critical? This code is guaranteed to blow up with a different heap implementation, if only in a Debug build. And besides, nobody would notice a fix just by looking at the game's rendered output…

Looks like we have to introduce a fourth category of weird code, in addition to the previous bloat, bug, and quirk categories, for invisible internal issues like these. Let's call it landmine, and fix them on the debloated branch as well. Thanks to Clerish for the naming inspiration!
With this new category, the full definitions for all categories have become quite extensive. Thus, they now live in CONTRIBUTING.md inside the ReC98 repository.

With the new discoveries and the new landmine category, TH01 is now at 67 bugs and 20 landmines. And the solution for the landmine in question? Simplifying the 61 lines of the original code down to 16. And yes, I'm including comments in these numbers – if the interactions of the code are complex enough to require multi-paragraph comments, these are a necessary and valid part of the code.

While we're on the topic of weird code and its visible or invisible effects, there's one thing you might be concerned about. With all the rearchitecting and data shifting we're doing on the debloated branch, what will happen to the 📝 negative glitch stages? These are the result of a clearly observable bug that, by definition, must not be fixed on the debloated branch. But given that the observable layout of the glitch stages is defined by the memory surrounding the scene stage variable, won't the debloated branch inherently alter their appearance (= ⚠️ fanfiction ⚠️), or even remove them completely?

Well, yes, it will. But we can still preserve their layout by hardcoding the exact original data that the game would originally read, and even emulate the original segment relocations and other pieces of global data.
Doing this is feasible thanks to the fact that there are only 4 glitch stages. Unfortunately, the same can't be said for the timer values, which are determined by an array lookup with the un-modulo'd stage ID. If we wanted to preserve those as well, we'd have to bundle an exact copy of the original REIIDEN.EXE data segment to preserve the values of all 32,768 negative stages you could possibly enter, together with a map of all relocations in this segment. 😵 Which I've decided against for now, since this has been going on for far too long already. Let's first see if anyone ever actually complains about details like this…

Alright, time to start the anniversary branch by rendering everything at its correct internal unaligned X position? Eh… maybe not quite yet. If we just hacked all the necessary bit-shifting code into all the format-specific blitting functions, we'd still retain all this largely redundant, bad, and slow code, and would make no progress in terms of portability. It'd be much better to first write a single generic blitter that's decently optimized, but supports all kinds of sprites to make this optimization actually worth something.
So, next research question: How would such a blitter look like? After I learned during my 📝 first foray into cycle counting that port I/O is slow on 486 CPUs, it became clear that TH04's 📝 GRCG batching for pellets was one of the more useful optimizations that probably contributed a big deal towards achieving the high bullet counts of that game. This leads to two conclusions:

• master.lib's super_*() sprite functions are slow, and not worth looking at for inspiration. Even the 📝 tiny format reinitializes the GRCG on every color change, wasting 80 cycles.
• Hence, our low-level blitting API should not even care about colors. It should only concern itself with blitting a given 1bpp sprite to a single VRAM segment. This way, it can work for both 4-plane sprites and single-plane sprites, and just assume that the GRCG is active.

Maybe we should also start by not even doing these unaligned bit shifts ourselves, and instead expect the call site to 📝 always deliver a byte-aligned sprite that is correctly preshifted, if necessary? Some day, we definitely should measure how slow runtime shifting would really be…

What we should do, however, are some further general optimizations that I would have expected from master.lib: Unrolling the vertical loop, and baking a single function for every sprite width to eliminate the horizontal loop. We can then use the widest possible x86 MOV instruction for the lowest possible number of cycles per row – for example, we'd blit a 56-wide sprite with three MOVs (32-bit + 16-bit + 8-bit), and a 64-wide one with two 32-bit MOVs.
Or maybe not? There's a lot of blitting code in both master.lib and PC-98 Touhou that checks for empty bytes within sprites to skip needlessly writing them to VRAM:

uint8_t left_half = ((uint8_t *)(sprite))[0];
uint8_t right_half = ((uint8_t *)(sprite))[1];
if(right_half != 0x00) {
	pokeb(VRAM_SEGMENT, (vram_offset + 0), left_half);
}
if(right_half != 0x00) {
	pokeb(VRAM_SEGMENT, (vram_offset + 1), right_half);
}

Which goes against everything you seem to know about computers. We aren't running on an 8-bit CPU here, so wouldn't it be faster to always write both halves of a sprite in a single operation?

uint16_t both_halves = ((uint16_t *)(sprite))[0];
pokew(VRAM_SEGMENT, vram_offset, both_halves);

That's a single CPU instruction, compared to two instructions and two branches. The only possible explanation for this would be that VRAM writes are so slow on PC-98 that you'd want to avoid them at all costs, even if that means additional branching on the CPU to do so. Or maybe that was something you would want to do on certain models with slow VRAM, but not on others?

So I wrote a benchmark to answer all these questions, and to compare my new blitter against typical TH01 blitting code:

2023-03-05-blitperf.zip And here are the real-hardware results I've got from the PC-9800 Central Discord server:

The key takeaways:

• Checking for empty bytes has never been a good idea.
• Preshifting sprites made a slight difference on the 286. Starting with the 386 though, that difference got smaller and smaller, until it completely vanished on Pentium models. The memory tradeoff is especially not worth it for 4-plane sprites, given that you would have to preshift each of the 4 planes and possibly even a fifth alpha plane. Ironically, ZUN only ever preshifted monochrome single-bitplane sprites with a width of 8 pixels. That's the smallest possible amount of memory a sprite can possibly take, and where preshifting consequently has the smallest effect on performance. Shifting 8-wide sprites on the fly literally takes a single ROL or ROR instruction per row.
• You might want to use MOVS instead of MOV when targeting the 286 and 386, but the performance gains are barely worth the resulting mess you would make out of your blitting code. On Pentium models, there is no difference.
• Use the GRCG whenever you have to render lots of things that share a static 8×1 pattern.
• These are the PC-98 models that the people who are willing to test your newly written PC-98 code actually use.

Since this won't be the only piece of game-independent and explicitly PC-98-specific custom code involved in this delivery, it makes sense to start a dedicated PC-98 platform layer. This code will gradually eliminate the dependency on master.lib and replace it with better optimized and more readable C++ code. The blitting benchmark, for example, is already implemented completely without master.lib.
While this platform layer is mainly written to generate optimal code within Turbo C++ 4.0J, it can also serve as general PC-98 documentation for everyone who prefers code over machine-translating old Japanese books. Not to mention the immediacy of having all actual relevant information in one place, which might otherwise be pretty well hidden in these books, or some obscure old text file. For example, did you know that uploading gaiji via INT 18h might end up disabling the VSync interrupt trigger, deadlocking the process on the next frame delay loop? This nuisance is not replicated by any emulators, and it's quite frustrating to encounter it when trying to run your code on real hardware. master.lib works around it by simply hooking INT 18h and unconditionally reenabling the VSync interrupt trigger after the original handler returns, and so does our platform layer.

So, with the pellet draw calls batched and routed through the new renderer, we should have gained enough free CPU cycles to disable 📝 interlaced pellet rendering without any impact on frame rates?

Well, kinda. We do get 56.4 FPS, but only together with noticeable and reproducible tearing in the top part of the playfield, suggesting exactly why ZUN interlaced the rendering in the first place. 😕 So have we already reached the limit of single-buffered PC-98 games here, or can we still do something about it?
As it turns out, the main bottleneck actually lies in the pellet unblitting code. Every EGC-"accelerated" unblitting call in TH01 is as unbatched as the pellet blitting calls were, spending an additional 17 I/O port writes per call to completely set up and shut down the EGC, every time. And since this is TH01, the two-instruction operation of changing the active PC-98 VRAM page isn't inlined either, but instead done via a function call to a faraway segment. On the 486, that's:

• >341 cycles for EGC setup and teardown, plus
• >72 cycles for each 16-pixel chunk to be unblitted.

This sums up to

• >917 cycles of completely unnecessary work for every active pellet, in the optimal 50% of cases where it lies on an even VRAM byte, or
• >1493 cycles if it lies on an odd VRAM byte, because ZUN's code extends the unblitted rectangle to a gargantuan 32×8 pixels in this case

And this calculation even ignores the lack of small micro-optimizations that could further optimize the blitting loop. Multiply that by the game's pellet cap of 100, and we get a 6-digit number of wasted CPU cycles. On paper, that's roughly ¹/₆ of the time we have for each of our target 56.423 FPS on the game's target 33 MHz systems. Might not sound all too critical, but the single-buffered nature of the game means that we're effectively racing the beam on every frame. In turn, we have to be even more serious about performance.

So, time to also add a batched EGC API to our PC-98 platform layer? Writing our own EGC code presents a nice opportunity to finally look deeper into all its registers and configuration options, and see what exactly we can do about ZUN's enforced 16-pixel alignment.
To nobody's surprise, this alignment is completely unnecessary, and only displays a lack of knowledge about the chip. While it is true that the EGC wants VRAM to be exclusively addressed in 16-bit chunks at 16-bit-aligned addresses, it specifically provides

• an address register (0x4AC) for shifting the horizontal start offsets of the source and destination to any pixel within the 16 pixels of such a chunk, and
• a bit length register (0x4AE) for specifying the total width of pixels to be transferred, which also implies the correct end offsets.

And it gets even better: After ⌈bitlength ÷ 16⌉ write instructions, the EGC's internal shifter state automatically reinitializes itself in preparation for blitting another row of pixels with the same initially configured bit addresses and length. This is perfect for blitting rectangles, as two I/O port writes before the start of your blitting loop are enough to define your entire rectangle.

The manual nature of reading and writing in 16-pixel chunks does come with a slight pitfall though. If the source bit address is larger than the destination bit address, the first 16-bit read won't fill the EGC's internal shift register with all pixels that should appear in the first 16-pixel destination chunk. In this case, the EGC simply won't write anything and leave the first chunk unchanged. In a 📝 regular blitting loop, however, you expect that memory to be written and immediately move on to the next chunks within the row. As a result, the actual blitting process for such a rectangle will no longer be aligned to the configured address and bit length. The first row of the rectangle will appear 16 pixels to the right of the destination address, and the second one will start at bit offset 0 with pixels from the rightmost byte of the first line, which weren't blitted and remained in the tile register.
There is an easy solution though: Before the horizontal loop on each line of the rectangle, simply read one additional 16-pixel chunk from the source location to prefill the shift register. Thankfully, it's large enough to also fit the second read of the then full 16 pixels, without dropping any pixels along the way.

And that's how we get arbitrarily unaligned rectangle copies with the EGC! Except for a small register allocation trick to use two-register addressing, there's not much use in further optimizations, as the runtime of these inter-page blit operations is dominated by the VRAM page switches anyway.

Except that T98-Next seems to disagree about the register prefilling issue:

Every other emulator agrees with real hardware in this regard, so we can safely assume this to be a bug in T98-Next. Just in case this old emulator with its last release from June 2010 still has any fans left nowadays… For now though, even they can still enjoy the TH01 Anniversary Edition: The only EGC copy algorithm that TH01 actually needs is the left one during the single-buffered tests, which even that emulator gets right.
That only leaves 📝 my old offer of documenting the EGC raster ops, and we've got the EGC figured out completely!

And that did in fact remove tearing from the pellet rendering function! For the first time, we can now fight Elis, Kikuri, Sariel, and Konngara with a doubled pellet frame rate:

With only pellets and no other animation on screen, this exact pattern presents the optimal demonstration case for the new unblitter. But as you can already tell from the invincibility sprites, we'd also need to route every other kind of sprite through the same new code. This isn't all too trivial: Most sprites are still rendered at byte-aligned positions, and their blitting APIs hide that fact by taking a pixel position regardless. This is why we can't just replace ZUN's original 16-pixel-aligned EGC unblitting function with ours, and always have to replace both the blitter and the unblitter on a per-sprite basis.
To completely remove all flickering, we'd also like to get rid of all the sprite-specific unblit ➜ update ➜ render sequences, and instead gather all unblitting code to the beginning of the game loop, before any update and rendering calls. So yeah, it will take a long time to completely get rid of all flickering. Until we're there, I recommend any backer to tell me their favorite boss, so that I can focus on getting that one rendered without any flickering. Remember that here at ReC98, we can have a Touhou character popularity contest at any time during the year, whenever the store is open!

In the meantime, the consistent use of 8×8 rectangles during pellet unblitting does significantly reduce flickering across the entire game, and shrinks certain holes that pellets tend to rip into lazily reblitted sprites:

To round out the first release, I added all the other bug fixes to achieve parity with my previously released patched REIIDEN.EXE builds:

• I removed the 📝 shootout laser crash by simply leaving the lasers on screen if a boss is defeated,
• prevented the HP bar heap corruption bug in test or debug mode by not letting it display negative HP in the first place, and
• restored 📝 the two animations during the Sariel fight that were lost to type confusion errors in the original game.

So here it is, the first build of TH01's Anniversary Edition: 2023-03-05-th01-anniv.zip Edit (2023-03-12): If you're playing on Neko Project and seeing more flickering than in the original game, make sure you've checked the Screen → Disp vsync option.

Next up: The long overdue extended trip through the depths of TH02's low-level code. From what I've seen of it so far, the work on this project is finally going to become a bit more relaxing. Which is quite welcome after, what, 6 months of stressful research-heavy work?